Synthesis of Bifunctional Ag@Porphyrin Nanostructures for Single‐Particle SERS Operando Tracking of Ferryl Intermediates in Iron‐Porphyrin Complexes

Metalloporphyrins have been extensively employed as model systems for mimicking heme enzymes and single-site catalysts. Understanding their catalytic mechanism at the molecular level, especially through in situ/operando characterization is of significant interest. Herein, we report the synthesis of bifunctional Ag@porphyrin core@shell nanocomposites (NCs) using Ag nanocubes and iron meso-tetra(4-carboxyphenyl) porphyrin (FeTCPP) via a facile noncovalent self-assembly method. The synthesized Ag@FeTCPP NCs inherit the advantages of ferric porphyrin regarding multienzyme-mimicking properties, while exhibiting exceptional surface-enhanced Raman scattering (SERS) activity. Using the disproportionation reaction of hydrogen peroxide as an example, single-nanoparticle SERS analysis of Ag@FeTCPP NCs facilitates the operando characterization of reactive intermediates involved in porphyrin-based catalytic cycles. In conjunction with the density functional theory (DFT) calculation and isotopic labeling experiment, three key intermediate species of ferric-hydroperoxo (Fe^III-OOH), ferryl-oxo porphyrin radical cation (Por^+•-Fe^IV═O), and ferryl-hydroxy (Fe^IV-OH) are clearly identified through dynamic SERS spectra. These results for the first time link spectroscopic characteristics to the theoretically expected water-assisted proton-coupled electron transfer mechanism. This work not only presents an efficient SERS model for exploring the nature and evolution of unstable reactive intermediates involved in porphyrin-associated catalytic systems, but also offers a generalizable design strategy for the fabrication of a novel iron-porphyrin catalyst.